In order to filter out harmonic currents which occur in converter circuits, one common practice has been to connect to the a-c system, a plurality of filter circuits, with one filter circuit tuned to each of the harmonic currents to be filtered out. The frequency of the harmonic currents occuring in such a converter circuit with a three-phase rectifier bridge are expressed by the following relation: EQU f = (k . p .+-. 1) . f.sub.1,
Where k = 1, 2, 3, 4 . . . ; p = 6 and f.sub.1 = the line frequency.
From this relationship, the lowest harmonic is the fifth harmonic.
If a filter arrangement is provided in which individual filters are used to filter out the individual frequencies of the harmonic currents, the filter circuits become very expensive.
In high-voltage d-c transmission systems, it is common practice to use 12-pulse converters. Converter circuits of this nature have the advantage, if the circuits are exactly symmetrical, that the harmonic current obtained for an odd factor k cancel. Thus, with such converter circuits, the above described tuned filters are theoretically unnecessary. However, since it is not possible to make the converter circuit exactly symmetrical, odd harmonic currents in residual amounts still are present even in 12-pulse converter circuits. These residual components can lead to large disturbances if the system is in parallel resonance with the filters at the frequencies of these harmonic currents. Since the network impedence changes continuously as a function of frequency due to connecting and disconnecting actions at the load, parallel resonance between the system and the filters can be expected to occur over and over at certain times. For this reason, measures must be taken even with 12-pulse converter circuits to filter out the odd harmonic currents resulting from an odd factor k in the above equation. In some cases, in order to insure trouble-free operation, when using individual filters the cost of the filter portion of the installation exceeds that of the remainder of the converter circuits.
Thus, it can be seen that there is a need for an improved filter arrangement for use with converter circuits which insures trouble-free operation and is less costly than the filter circuits used in the prior art.